The present invention relates to a process for converting iron-containing non-ferrous matte, such as nickel and/or copper matte.
More particularly, the present invention relates to a conversion process using a top blowing/bottom stirring mechanism, in which charges of cold non-ferrous matte are added to a molten bath of matte, while the iron content is continuously or periodically removed from the molten bath surface as slag. The conversion reaction is effected by blowing an oxygen-containing gas onto the bath surface, while stirring from below with a non-reactive sparging gas.
Specifically, nickel and/or copper sulfide furnace mattes may contain iron in amounts in excess of 30% by weight. By employing the novel process claimed herein, the iron content can be efficiently reduced to below 10% in the case of nickel and nickel/copper matte, and completely removed in the case of copper matte. Moreover, a steady, concentrated stream of SO.sub.2 may be readily captured.
In general, the objective of a conversion process is to oxidize the iron sulfides in the matte to form iron oxides and to liberate sulfur dioxide, leaving matte comprising predominantly non-ferrous sulfides. In the initial stage, the removal of iron oxide is facilitated by the addition of a flux, such as silica in the case of nickel matte, which forms an immiscible iron silicate slag which may be skimmed from the top of the melt. In the case of copper matte, commonly used fluxes would be lime or silica. The slag may also contain other impurities which are oxidized in the process.
Traditionally, the oxygen-containing gas is injected into the molten bath via submerged tuyeres. This results in extreme wear of the tuyeres, due to the highly exothermic nature of the oxidation reaction, particularly at the point of injection. Attempts to protect the tuyeres by "shrouding" have been disclosed for nickel matte refining processes, e.g. in U.S. Pat. No. 4,045,215. However, this results in added expense due to the increasing complexity of the process, which requires tuyeres having concentric tubes so that protective coolant, such as fuel oil, natural gas or nitrogen may be blown in around the oxygen.
Another known process is the so-called "Mitsubishi" process, (see Nagano et al., "Commercial Operation of Mitsubishi Continuous Copper Smelting and Converting Process", International Symposium on Copper Extraction & Refining, 1976, pp. 439-57). In this method, the bath is top blown with oxygen by a complex mechanism, which utilizes a set of consumable lances fixed to a rotating member, as described in U.S. Pat. No. 3,968,956. As the lances rotate, they are lowered into the hot zone to achieve maximum efficiency. However, the extreme heat quickly consumes the lances, which thus require replacement on a regular basis. Furthermore, the rotating member is plagued by a number of drawbacks owing to its complexity. For example, the near impossibility of obtaining a proper seal between the rotating member and furnace shell leads to hazardous dusting. Also, a build-up of material must be constantly removed.
These obstacles had been overcome by the use of a "top blowing/bottom stirring" mechanism, as described with respect to the converting of white metal copper by Marcuson et al. in U.S. Pat. No. 4,830,667. In the Marcuson process, an oxygen containing gas is blown into the surface of the molten bath while the bath is stirred from below with a non-reactive sparging gas, such as nitrogen. The stirring of the bath continuously supplies fresh reactants to the surface, where oxidation can readily take place. It has now been found that this efficient mechanism can be successfully applied as part of a novel process for the conversion of FeS-containing non-ferrour matte.
Furthermore, in contrast to previous understanding, it has now been demonstrated that a top blowing/bottom stirring conversion process can be quite successful in accepting non-ferrous matte as its sole feed. It had been believed that the production of iron-containing slag as a result of cold matte addition would interfere with the conversion reactions taking place in the melt. However, it has been surprisingly found that the reaction proceeds efficiently so long as the slag thickness is maintained below a certain threshold.